To reproduce this.
1- Set port rules on both your router and windows firewall for TCP and UDP.
2- Verify that your port is open using one of those ports checking websites.
2- Set Tixati incoming protocol to UDP Only.
3- Start downloading a torrent with many peers.
4- Restart the incoming connections counter (by restarting the program for example).
5- After 15 minutes, write down the number of incoming connections.
6- Change Tixati incoming protocol to TCP only.
7- After 3 minutes, write down the number of incoming connections.
8- Compare the results of TCP and UDP.

By the way, i tested v2.88 & 2.87 / Windows 7 SP1 32bit. I tried many ports numbers, and tested another torrent client (with only UDP allowed) before reporting this problem.

The problem vanished after rebooting my computer!